Knotless splicing systems tend to progressively replace the classical knotters in all fields of textile. The first condition to be met for obtaining a joint in a yarn which is strong and not visible at the same time, is to properly loosen and unravel the ends of the yarn to be joined. Theoretically, since the yarn results from a degree of twisting of the fibers, it is sufficient to untwist the yarn while holding it firmly and to remove the fibers which get loose from the section where the yarn is held. Although this technique is quite valid for yarns obtained by spinning on a ring spinning frame, it does no longer operate in the case of yarn produced by the "open-end" technique or for two-plyed yarns. Furthermore, it should be remembered that yarns can be obtained with a right-hand or a left-hand twist. This explains why it is difficult to loosen and unravel certain types of yarn and, practically, no apparatus or method exists which can loosen all types of yarn.
Among the proposed solutions, one should particularly distinguish those relying on mechanical effects from those relying on air dynamics. Such solutions are disclosed in DE-C2-2,954,426 and in DE-C2-2,939,481. In both documents, the yarn is introduced into a duct in which a tangential air flow is provided to untwist the yarn and separate the loosened fibers from each other, i.e. to effect unravelling.
As previously mentioned, this untwisting mode is only valid, at best, for yarns achieved according to the classic ring spinning frame technique but is ineffective for other types of yarn. Moreover, in practising the methods of these documents, the twisting rotation of the yarn, in S or Z, must be known, so that the air-dynamic action is applied to the yarn in a direction opposite to that of untwist. A particular problem is therfore present in the case of two plyed yarns which can consist of S-twisted yarns which are thereafter Z-twisted, or the converse. This is the reason why a method has been proposed in EP-A1 0 053 093 which consists of reducing the rubbing forces existing between the fibers by subjecting them to vibrations and by simultaneously exerting an axial stress for separating the fibers after loosening the yarn. It has been shown that this technique makes it possible to loosen and unravel yarns known to be difficult, such as "open-end" yarns and two plyed yarns.
It has been shown that the vibrating or pulsed member contributes to improving continuous methods each time the yarn under consideration has fibers which are not parallel, for instance in the case of two plyed yarns or open-end yarns. Indeed, when some of the fibers are not parallel, they can tangle up and a periodically interrupted action is then more efficient than a static action which, in fact, promotes tangling up. This is the reason why better results are obtained with a vibrating member present in the air flow duct in which the yarn to be loosened has been introduced.